1. Field of the Invention
This invention relates to the fluid catalytic cracking (FCC) of crude oil fractions to improve the yield of desired products.
2. Review of the Art
Fluid catalytic cracking has long been known as a technique for reforming crude oil fractions so as to improve the yield of useful hydrocarbons and so as to tailor the output of a refinery to provide different types such useful hydrocarbons in proportions appropriate to the demand for such different types of hydrocarbons. Usually the ruling factor in such tailoring is the demand for high octane gasoline fractions, the demand for which has been increased by the phasing out of the use of lead compounds as octane improvers in gasolines.
A number of factors interact in determining the product mix produced by a conventional fluid catalytic cracking unit, including the nature of the feedstock, the activity of the catalyst, and the riser temperature in the cracking unit. There is substantial operational interaction between these factors. For example, an increased riser temperature promotes hydrogen transfer reactions which create cyclic and isomeric molecules which produce improved octane levels in gasoline range fractions. Such an increased riser temperature also promotes the deposition of coke on the catalyst which reduces catalyst activity, and decreases production of desired products. The conditions under which such units operate thus inevitably represent a compromise. The heavier the feedstock the more difficult it becomes to reach a satisfactory compromise.
One approach to providing an improved compromise is to operate the unit in a manner which minimizes any increase in coking reactions as cracking conditions are optimized to produce a desired product mix. Coking reactions can be regarded as a subset of the diverse reactions which can occur on or within catalyst particles during cracking. They are dependent upon the reaction environment and upon the nature of the feedstock being sent to the cracking unit. Coking reactions are dehydrogenation reactions which produce a tar-like oil product and considerable quantities of gas, and represent unwanted cracking reactions which reduce the yield of wanted products and foul the catalyst.
The ideal environment for such coking reactions is a liquid phase at approximately 425.degree. C., in the substantial absence of catalytic activity. These are the conditions which exist at the surface of a catalyst particle in contact with liquid feedstock prior to vaporization of the latter. Catalytic cracking on the other hand is ideally a vapour phase reaction in which individual oil molecules enter pores in the catalyst and reach active sites which promotes hydrogen transfer reactions leading to cracking of the molecule. The actual cracking reactions produce little or no coke for most molecules capable of entering the catalyst pores, but instead tend to produce molecules in the C.sub.6 -C.sub.15 range. In order to maximize cracking and minimize coke formation, the oil feedstock should thus be vaporised as rapidly as possible.
In an article `FCC heat balance critical for heavy fuels` by J. L. Mauleon and J. C. Courcelle, Oil and Gas Journal, Oct. 21, 1985, pages 64 et seq., the authors discuss the interaction of these and other factors, and at page 65 provide a table illustrating the theoretical relationship between oil droplet size and vaporization time, for oil droplet sizes of 500, 100 and 30 microns. It will also be noted that as droplet size decreases, the decrease in vaporization time as shown in the table is initially very great for the change from 500 to 100 microns, but much less for the further decrease from 100 to 30 microns. This may be related to the vaporization mechanism. With large oil particles, individual catalyst particles do not have sufficient thermal capacity fully to vaporize an oil particle. As the heat required to vaporize an oil particle decreases relative to the thermal capacity of the catalyst particles, the vaporization time initially drops very rapidly. The table given in the article appears to have been constructed upon the hypothesis that once the heat of vaporization of the oil particles is small relative to the thermal capacity of the catalyst particles, the rate of vaporization remains primarily controlled by boundary layer effects so that little further improvement is obtained. In brief, a boundary layer (which may be defined as that layer within which 99% of the temperature difference between the oil and the catalyst occurs) of liquid in immediate contact with the catalyst particle vaporizes, and the vapour produced inhibits conductive heat transfer to the remainder of the liquid, so that the primary heat transfer mechanism is convection within the liquid phase. On this hypothesis, once the particle size of the oil droplets becomes small enough that most of the oil particles are sufficiently small relative to the catalyst particles that a typical single catalyst particle has sufficient thermal capacity to vaporize a typical single oil particle, there is little advantage in further oil particle size reduction.
It can be calculated that the amount of energy contained in a catalyst particle will be capable of providing an oil particle of between 40 and 55 microns with enough energy to vaporize it. The exact size of the oil particle which can be vaporized will vary with the temperature of the oil and catalyst, as well as the thermal properties of both. In most cases, if oil particles have a diameter of less than 40 microns, a single catalyst particle will have the necessary thermal capacity to vaporize an oil particle.
Processes in which reduction towards this level of oil particle size are effected to improve vaporization time and reduce coking have been proposed and implemented. Thus in two further articles, in Oil and Gas Journal, "Total introduces new FCC process" (Oct. 11, 1982) and "Resid puts FCC process in new perspective" (Oct. 4, 1982) both by Robert Dean, J. L. Mauleon and Warren Letzsch, such a process is discussed, although no specifics of particle size are disclosed. More details of certain aspects of the process and apparatus are to be found in U.S. Pat. Nos. 4,427,537 and 4,434,049 issued to Dean et al. These patents disclose an FCC process in which the oil is atomized to form droplets ranging in size between 10 and 500 microns, although the actual size distribution within this range and the average particle size are not discussed.